1. Field of the Invention
The present invention relates to an output current detecting circuit losing little electric power, an output current detecting circuit being scarcely influenced by power supply voltage variations and temperature variations, and a transmission circuit equipped with one of the output current detecting circuits.
2. Description of Related Art
A home bus system (HBS) exists as a communication standard between household electrical appliances. The HBS includes a standard regulating the use of twisted-pair lines for a transmission path and the use of an alternate mark inversion (AMI) coded signal (hereinafter referred to as an AMI signal) for the transmission of a digital signal on the transmission path. An AMI signal is composed of three values of zero, plus, and minus, and data is transmitted in the way of expressing logic “0” by a zero and logic “1” by alternating the polarity of the signal in communication using the signal. A transmission waveform hereby becomes close to that of an alternating current signal, and the AMI signal has the advantages of being tolerant of noise and of enabling stable data transmission. In addition, the polarities of the logic “1” are positive and negative polarities to the electric potential of the logic “0,” and the electric potential of the logic “0” is not limited to 0 V. For example, 5 V may be selected as the electric potential of the logic “0.”
An HBC driver/receiver integrated circuit (IC) (semiconductor integrated circuit) has conventionally been provided as a device that is mounted on a piece of equipment constituting a system, to which HBS is applied, and bears the communication function between pieces of equipment. A transmission circuit generating an AMI signal to transmit the generated AMI signal onto a transmission line is incorporated into the IC in addition to a receiving circuit judging the logic level of an AMI signal on a transmission line to reproduce received data, and the transmission circuit is equipped with an output drive circuit to drive the transmission line and a transmission gate control circuit controlling the output drive circuit on the basis of transmission data (see, for example, Japanese Patent Application Laid-Open Publications No. H 5-315852 and No. 2007-195007). The output drive circuit here uses a power transistor capable of flowing a large current as the output transistor thereof in order to enable driving a transmission line, the length of which is sometimes several tens of meters or more.
In a system to which HBS is applied, several tens of pieces of equipment are sometimes connected to one transmission path. For example, several tens of pieces of indoor equipment (expanders and heat exchangers) are sometimes connected to one or several pieces of outdoor equipment (compressors and radiators) through a transmission path in an air conditioning system of a building, and an HBC driver/receiver IC is mounted on each piece of equipment. In such an HBS system, a situation in which the driver/receiver ICs of a plurality of pieces of equipment simultaneously perform transmission sometimes occurs. To put it concretely, there may be a case where, when the transmission circuit of a certain driver/receiver IC tries to output a positive logic signal, the transmission circuit of another driver/receiver IC happens to try to output a negative logic signal.
In such a case, it is apprehended that a very large current flows through the output transistor of the transmission circuit trying to output a positive logic signal and the output transistor may be broken in some cases. Accordingly it is preferable that a current detecting circuit detecting a current flowing through an output drive circuit is incorporated, and that, if the current detecting circuit detects that a current equal to or more than a predetermined value flows through the output drive circuit, the transmission gate control circuit stops the output operation of the output drive circuit. The inventors of the present invention devised a circuit shown in FIG. 6 as a transmission circuit having such a function, and examined the circuit.
The circuit shown in FIG. 6 is composed of an output drive circuit 11 driving a transmission line to output a AMI-coded data signal, a gate control circuit 12 generating control signals for performing the on-off control of the respective transistors Q1 and Q2 of the output drive circuit 11 on the basis of transmission data, and an output current detecting circuit 13 including a comparator comparing the voltage of a current detecting resistor Rs connected between a power source voltage terminal VDD and the output transistor Q1 with a reference voltage Vref to detect whether a current equal to or more than a predetermined current value (excess current) is flowing or not.
The output drive circuit 11 is composed of a p-channel type power metal oxide semiconductor (MOS) transistor Q1 and an n-channel type power MOS transistor Q2, each made of an insulated-gate field-effect transistor (hereinafter referred to as a MOS transistor), both connected in series with each other between the power source voltage terminal VDD and a ground potential point GND. Furthermore, the circuit of FIG. 6 is configured in such a way that, when a current equal to or more than the predetermined current value flows through the output transistor Q1 and the voltage dropped by the current detecting resistor Rs becomes lower than the reference voltage Vref, the output current detecting circuit 13 transmits a detection signal to the gate control circuit 12, and that the gate control circuit 12 controls both the output transistors Q1 and Q2 into their turned-off states to prevent the flow of the excess current.
Because a relatively large current flows through the current detecting resistor Rs (hereinafter referred to as a sensing resistor) provided in series with the output transistor Q1 in the output current detecting circuit of FIG. 6, the power loss thereof is large and the power consumption thereof becomes much. Consequently, when a chip temperature rises owing to the heat generation of the sensing resistor Rs to exceed a package allowable temperature, it is apprehended that the device is broken. Although the power loss of the sensing resistor Rs can be reduced, here, by using a low resistance element, it is difficult by the present process technique to obtain a highly accurate low resistance element on a semiconductor chip on which the output current detecting circuit is formed, and, if the resistance value of the sensing resistor Rs disperses, an excess current detection level results in dispersing.
Furthermore, because the p-channel type MOS transistor, having a device size larger than that of the n-channel type MOS transistor of the same drive power, is used as the output transistor Q1 in the output current detecting circuit of FIG. 6, the output current detecting circuit has the problem in which the occupation area of the output circuit, the chip size thereof by extension, is large. In addition, the invention pertaining to a detection circuit configured to be able to detect an excess current without causing any large power losses by providing a current detecting transistor connected to an output transistor through which a large drive current is flown in a current mirror connection is described in, for example, Patent Literatures 1 and 2.